NEWARK—Bridget Kelly told her boss, New Jersey Gov. Chris Christie, about a plan to close lanes to the George Washington Bridge a month before the September 2013 traffic jams in Fort Lee, N.J., she testified Friday.

Ms. Kelly, who is accused of closing the lanes as political payback, told jurors she gave Mr. Christie the heads-up after being asked to by another Christie associate. The associate, David Wildstein, had informed her that...